Hi and welcome to SambaPOS,
I have the exact same setup, but I must tell you that I've done it a long time ago, so I will try to help as much as possible.
My 1st question to you will be: The Server you setup, what is it called on the Network? I assume you called that pc Server since you say Server=hostname.
On you server, set Local Settings -> Data Connection String = data source=localhost\SQLEXPRESS
(Not data source=Server\SQLEXPRESS, however, it should work should the pc name be Server)
On your clients you might need to set Data Connection String to: data source=[Server Computer Name]\[SQL Express Instance Name]; user id=[User Name]; password=[Password]
(I had to use this to get it to work)
I will get you the exact setting I used when I get back to work. +- 12 hours from now
But I hope this points you in the right direction.